Agreed that you should keep away from property. You already have an investment in property.
The best return from your money is most likely to be in shares held via a pension fund, so max your contributions to that.
Age: mid 40s
Co-mortgagee age: early 40s
Both single.
Are you happy enough with being a co-owner in the longer term? Have you or your co-owner any plans to go your separate ways and buy your own properties?
Even if you are, things can change. So you definitely should not tie yourself into an investment property which would be difficult to sell quickly and where the mortgage would limit your capacity to buy.
A few possible developments:
1) Your co-owner meets someone and wants to buy you out or sell the property. With a decent amount of cash and no borrowings, you may be able to buy him out and retain the tracker mortgage. (If you have an investment property, you might not qualify for the mortgage.)
2) You meet someone and want to buy a house together. With a decent amount of cash and not tied into an investment property, and a very profitable investment property, you should be able to do so. (If you have an investment property, your options would be limited)
3) There is a remote chance that the lender might sell on the tracker to a vulture fund who might do a deal with you for early repayment of the mortgage.
There are variations of the above as well. You both want out at the same time. You buy out your co-owner and then transfer the tracker to a new home.
Although you may have no such plans at the moment, circumstances change very quickly. The best way to exploit any such changes is to be flexible. Plenty of cash and low borrowings.
Brendan